“Retroreflectivity” means the ability, if engaged by a beam of light, to reflect that light substantially back in the direction of the light source. Films, for example, transparent adhesive-backed overlay films, having covert retroreflective patterns have been used to authenticate security articles (e.g., passports, identification badges). Typically, such overlay films have covert retroreflective patterns that are not readily legible under conditions of diffuse illumination, but become readily legible if viewed in retroreflective mode (e.g., with a flashlight or specialized device such as a retroviewer).
Depending on the application, the addition of covert color to retroreflective articles may provide additional security features (e.g., as in the case of authentication of passports or identification badges), or novel visual effects (e.g., as used in the design and production of graphic articles).
As used herein, the term “retrochromism” refers to the ability of an object, or region of an object, if viewed in retroreflective mode (i.e., with the illumination source substantially co-linear with the line of sight, typically forming an angle of reflection of about 10 degrees or less), to exhibit a reflected color different from the color exhibited if the object or region is viewed in other than retroreflective mode. Various constructions that exhibit retrochromism are known. For example, in one such construction high refractive index glass beads are partially embedded in a layer of or layers of material(s) (e.g., including a dielectric mirror). A change in color of retroreflected light results from the interaction of light, the bead, and the layer(s) into which it is partially embedded. Such articles may be prone to loss of, or irregularity in, their retrochromism due to coating thickness variations of the layer(s) and differences in the depth of penetration of the beads into the layer(s). Such articles may further be prone to damage caused by flexing the article during usage, or if used as heat-shrink tamper indicating films.
To overcome such drawbacks, it would be desirable to have retrochromic articles that are not subject to such variables. It would also be desirable that such articles have covert retrochromic patterns comprising multiple retroreflective colors.